The present invention relates to a drive apparatus for use in a four-wheel driven type motor vehicle.
With a viewpoint of safety and diversification, a four-wheel drive system (which will be referred hereinafter to as 4WD) and a traction control system (which will be referred hereinafter to as TRC) are being widely introduced in motor vehicles. In a conventional TRC, when the rotational speed of the driven wheel becomes higher than the rotational speed of the non-driven wheel, which does not slip irrespective of acceleration of the motor vehicle, a decision is made that the acceleration slip occurs in the driven wheel, thereby activating traction control. Accordingly, the TRC is generally applied to two-wheel driven type motor vehicle (which will be referred hereinafter to as 2WD vehicle). On the other hand, a conventional 4WD vehicle is equipped with a power transmission to transfer engine power to the front, rear, left and right wheels. The power transmission comprises a clutch, a transmission (speed change gear), a propeller shaft, a differential, an axle shaft and others. The respective wheels are mechanically coupled through the power transmission to the engine so as to be rotationally driven due to the power generated by the engine. Because of driving the respective wheels, the 4 WD can double the driving force as compared with the 2WD, thereby improving the travelling safety, running performance and others in travelling on a road surface having a low (small) friction or a large travelling resistance.
There is a problem which arises with the conventional 4WD, however, in that, when the driving forces of all the wheels exceed the gripping force of the road surface due to rapid acceleration or the like, the acceleration slips occur in all the wheels so as to considerably deteriorate the travelling safety of the motor vehicle. Thus, to maintain travelling safety, it is preferable that the motor vehicle be equipped with both the 4WD and TRC. However, as described above, the conventional TRC is arranged to detect the occurrence of the acceleration slip of the drive wheel by the comparison between the rotational speeds of the driven wheel and the non-driven wheel, thereby making it difficult to simply combine both the 4WD and TRC. That is, for the TRC operation, the 4WD state is required to be switched to the 2WD state (the 4WD is not compatible with the TRC). Thus, the motor vehicle requires a complex and expensive absolute vehicle speed meter which can detects the occurrences of the acceleration slips in all the wheels.